Method for continuous heat-treatment of metals under argon atmosphere

ABSTRACT

In heat-treating a metallic alloy containing volatile metals such as zinc, manganese, chromium, aluminium, and so on in a tunnellike continuous furnace, argon is employed as a furnace atmosphere so that a pressure selectively produced by the argon atmosphere in the furnace is utilized for preventing the volatile metals from evaporating out of the metallic alloy during its heat-treatment. The argon employed as the furnace atmosphere and having a specific gravity higher than the air prevents also the air from entering into the furnace, and works also to purge the air which has happened to come into the furnace together with the metallic alloy, whereby the furnace atmosphere is kept constantly inert.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a novel method for continuously heat-treating metals under an inert gas atmosphere within a tunnellike continuous furnace. In this invention, the metals are particularly those alloys which contain one or more volatile metals such as zinc, manganese, chromium, aluminium, and so on. These metals may also be termed vaporizable, or as those which will easily be sublimated. And, the inert gas atmosphere is primarily argon.

[0002] Heat-treatment of such alloys may be exampled by brazing stainless steel sheets or plates with a nickel-base solder containing chromium. This kind of heat-treatment can scarcely be performed under vacuum, because the solder will be dechromized. This type of brazing can neither be achieved under a nitrogen atmosphere, because chromium contained in the solder will be nitrided. Thus, this kind of brazing has been manually conducted piece by piece in the air.

[0003] When it is desired to braze a batch of stainless steel parts at a time, or to braze them successively in large quantities, they can be brazed with oxygen free copper. This is, however, against a modern demand that physical characteristics, with which brazed portions of stainless steel parts are to be afforded, specifically their thermostable properties have to be remarkably improved. Such demand will be satisfied if a solder contained with chromium can be employed, and if brazing can be made effectively in large quantities. This supposition has been practically unlike, as described above.

[0004] Though an example in which stainless steel plates were successively brazed under an argon atmosphere, is described in Japanese Preliminary Patent Publication No.06-238433, inventor and applicant of which are same to those of this application, the solder employed there was nothing but oxygen free cupper.

[0005] And, in another Japanese Preliminary Patent Publication No.2000-273528 of the same inventor and applicant, there is described a method in which argon was effectively used as a furnace atmosphere for heating metals. While this publication teaches that chrome stainless steel was sintered without oxidation, it neither refer to at all the above-mentioned problems that such metallic alloy would readily be dechromized or nitrided when it is heated, nor it concern how to solve them.

SUMMARY OF THE INVENTION

[0006] In view of the aforementioned backgrounds, it is an object of this invention to provide a method for heating metallic alloy parts and articles containing one or more volatile metals such as zinc, manganese, chromium, and aluminium, vapor pressures of which under different temperatures are given in FIG. 2, so that they can effectively undergo various heat-treatments within a tunnelike continuous atmosphere furnace successively in large quantities, and so that such metals shall neither be vaporized or sublimated from their metallic alloy parts and articles, nor they shall be subjected to chemical reactions such as nitriding.

[0007] In this invention, the inventor has observed that in a tunnellike continuous furnace which is operated generally under an atmosphere of 0.1 to 0.5 Pa, its atmospheric pressure within the furnace can be elevated, when an argon gas is employed as the furnace atmosphere, because it has a specific gravity of 1.783Kg/m³ which is considerably higher than the specific gravity of air of 1.293Kg/m³. The argon furnace atmosphere which is inert, shall not cause a chemical change of metal alloy parts and articles to be heat-treated. Even when they contain volatile zinc, manganese, chromium, and/or aluminium, a high furnace atmospheric pressure sustained by argon shall prevent these volatile metals from vaporing from them.

[0008] Such high furnace atmospheric pressure can easily achieve the sealing of furnace inlet and outlet against the outer atmosphere, while such sealing has been a problem. In addition, such high furnace atmospheric pressure makes it easy to purge out from a furnace those airs and other disturbance which are unavoidably brought into the furnace together with parts or articles to be heat-treated in the furnace. Foreign materials such as machine oils which are burnt out from the parts or articles, shall also be discharged out easily from the furnace.

THE DRAWINGS

[0009]FIG. 1 is an explanatory sectional view of a tunnellike continuous furnace which can advantageously be employed for carrying out this invention method;

[0010]FIG. 2 is a graph showing vapor pressures of volatile metals including zinc and others in relation with temperatures; and

[0011]FIG. 3 is a view similar to FIG. 1, showing another tunnellike continuous furnace.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] In the following Examples 1, 2, 3, and 4, the furnace shown in FIG. 1 was employed.

[0013] As shown in FIG. 1, the continuous furnace has a heating chamber 1, and a preheating chamber 2 and a cooling chamber 3 located at either side of the heating chamber. The chambers are connected each other to make a tunnellike furnace. The furnace has an inlet 4 and an outlet 5 adjacently to its both ends, and forms as a whole and at its cross section a U-shape with the heating chamber as its base. Walls of the heating chamber 1 are preferably made from carbonous materials, so that when oxygen or oxygen compounds get casually mixed with a furnace atmosphere as a disturbance thereof, they shall be reacted with the carbonous walls, converted to carbon monoxide, and partly or wholly adsorbed into the walls, whereby an oxygen partial pressure in the furnace atmosphere can constantly be kept extremely low.

[0014] An endless mesh belt 6 which is also made preferably from carbonous materials, circulates within the furnace. It is moved by driving rollers 7 in the direction illustrated by arrows. A return passage 8, through which the unloaded belt 6 travels, is also hermetically sealed against the air, so that the argon gas which is supplied into the furnace from a supply pipe 9 located upward the heating chamber 1, can be sealed totally in the furnace.

[0015] While the inlet 4 works as a charging opening and the outlet 5 as a discharging opening for parts or articles which are to be or have been heat-treated, these openings which stand erect above a general height of the furnace or at least above the heating chamber 1, work also as heads having a height H above a horizontal level of the loaded belt 6.

[0016] When said height H (mm) of heads 4,5 is selectively varied, the argon pressure (Pa) inside the furnace varies accordingly as follows. Height of Heads: 300 600 1200 2400 4800 Furnace Inner Pressure: 5.0 10.0 19.3 38.5 77.2

EXAMPLE 1

[0017] SUS304 stainless steel sheets containing 18% of Cr and 8% of Ni were employed as test pieces. The test pieces were brazed with nickel-based solder BNi-5 containing 19% of Cr at 1,200° C. In this instance, the height of heads 4, 5 were set 300mm (corresponding furnace inner argon pressure: 5.0 Pa). The test pieces thus brazed were immersed in deionized water with 5 wt % of NaCl. Even when they were in the water for 96 hours, no rust was found on them. They did not change color.

[0018] The same test pieces which had been brazed at the conditions same to the above but within a vacuum furnace, were immersed in the same testing water. Only at one hour after they were in the water, rust was observed on them.

[0019] EXAMPLE 2

[0020] Test pieces were made by molding powders of stainless steel SUS447J1 containing 30% of Cr. They were sintered at 1,300° C. in the furnace with the heads 4, 5 of 300 mm height (viz., under the argon furnace atmosphere of 5.0 Pa).

[0021] The test pieces which had been sintered, were immersed in deionized water with 5 wt % of NaCl, resulting in that no rust was observed even after the immersion for 46 hours. They did not change color.

[0022] However, rust was found on those test pieces which had been heat-treated in a vacuum furnace, after they were immersed for 30 minutes in the same testing water.

[0023] EXAMPLE3

[0024] Brass sheets containing 30% of zinc were brazed in the furnace at 700° C. with a BAg-7 solder containing 18% of zinc.

[0025] Heights of the heads 4, 5 were selected to:

[0026] (1) 300 mm (viz., under the argon furnace atmosphere of 5.0 Pa),

[0027] (2) 1200 mm (viz., under the atmosphere of 19.3 Pa), and

[0028] (3) 4800 mm (viz., under the atmosphere of 77.2 Pa).

[0029] Brazing results under these (1), (2), and (3) conditions were visually observed, as follows.

[0030] Under the above (1) condition, zinc had been evaporated from the solder, whereby its melting point became higher, resulting in that it could not melt thoroughly. The brass sheets became reddish, because of dezincing phenomena.

[0031] Under the above (2) condition, the solder melted completely whereby brazing was achieved, though the brass sheets changed color.

[0032] And, under the above (3) condition, the solder melted completely, resulting in achieving fine brazing, and the brass sheets did not change color.

EXAMPLE 4

[0033] Powders of low nickel austenitic stainless steel containing 30% of Mn were molded under pressure to test pieces. They were heated at 1,300° C. in the furnace with the heads of a 600 mm height or higher (viz., under the argon furnace atmosphere of 10.0 Pa or higher). They were sintered well and were not rid of Mn.

[0034] The method of this invention was performed by another tunnellike continuous furnace which is illustrated in FIG. 3. In FIG. 3, those parts which are same to those of FIG. 1, are represented by the same reference numerals.

[0035] In this another continuous furnace, an inlet 4′ is made of a passage which has an incidence angle of α and forms a head of height H, while an outlet 5′ is made of another passage which has a corresponding reflection angle of α′ and forms a head of the same height H. In this another embodiment of the furnace for performing this invention, a return path of the belt 8′ may not necessarily be hermetically sealed from the air.

[0036] When the inlet and outlet passages 4′, 5′ of 1 m each were slanted with angles of said α and α′, they attained heads of a height of 267 mm, which height is sufficient to give an argon furnace pressure for performing this invention.

[0037] This type of inlet and outlet passages can advantageously be employed to readily charge and discharge metallic alloy articles into and from the furnace. It is matter of course that heads of a higher height, that is, a higher argon furnace atmosphere may be obtained by increasing the slanting angles and/or elongating the inlet and outlet tubular passages 4′ and 5′.

[0038] As described above, alloy articles containing volatile metals such as zinc, manganese, chromium, aluminium, and the like can be subjected to various heat-treatments positively and continuously in accordance with this invention. Since a furnace atmosphere of a high pressure is readily attained in this invention, it becomes easier to shelter an inlet and outlet of a tunnellike continuous furnace from the air, resulting in avoiding polluting the furnace atmosphere by the air. Accordingly, consumption of argon atmosphere gas is much saved. These are additional technical and economic advantageous aspects of this invention method. 

1. A method for heat-treating a metallic alloy, which comprises employing argon as an atmosphere of a tunnellike continuous heat-treating furnace, and heat-treating, under a pressure selectively produced by the argon atmosphere in the furnace, the metallic alloy containing one or a plurality of volatile metals such as zinc, manganese, chromium, aluminium, and so on.
 2. The method as claimed in claim 1, in which the metallic alloy is a stainless steel and a nickel based solder containing chromium, and the heat-treating is for brazing said steel with said solder.
 3. The method as claimed in claim 1, in which the metallic alloy is an article made by molding stainless steel powders, and the heat-treating is for sintering said article.
 4. The method as claimed in claim 1, in which the metallic alloy is brass containing zinc and a silver solder containing zinc, and the heat-treating is for brazing the brass with said silver solder.
 5. The method as claimed in claim 1, in which the metallic alloy is an article made by molding stainless steel powders containing manganese, and the heat-treating is for sintering said article.
 6. The method as claimed in claim 1, 2, 3, 4, or 5, in which the pressure selectively produced by the argon atmosphere in the furnace is in a range from about 5.0 Pa to about 77.2 Pa.
 7. The method as claimed in claim 1 or 6, in which the pressure selectively produced by the argon atmosphere in the furnace is changed by the height of an inlet and outlet which are for charging and discharging the metallic alloy into and from the tunnellike continuous heat-treating furnace and which work as outstanding heads.
 8. The method as claimed in claim 1 or 6, in which the pressure selectively produced by the argon atmosphere in the furnace is raised by slantingly elevating an inlet and outlet which are for charging and discharging the metallic alloy into and from the tunnellike continuous heat-treating furnace. 